Mushroom extracts have long been used in Asia as immunopotentiating agents. Many studies, mostly conducted in Japan and China, have shown that ingestion of mushroom extracts can induce tumor regression or stabilization in both animal models of common cancers as well as in cancer patients. However, there have been few investigations evaluating the potential mechanism of the anti-tumor effect of these agents. The proposed study will use the neu transgenic (tg) mouse, a model of ER-, HER2+ breast cancer, to study the immunomodulatory effects of protein-bound polysaccharide K (PSK), a commercially available mushroom extract. In preliminary studies, we have found that PSK may induce both mobilization and maturation of dendritic cells (DC) into the draining lymph nodes resulting in the proliferation of antigen specific T cells in ova TCR-transgenic mice. In neu-tg mice, PSK inhibited tumor growth by over 60%. Intra- tumoral injection of PSK in neu-tg mice increased the number of mature DC in the tumor draining lymph nodes and markedly increased the CD8+ T cell infiltrate in the tumor bed. Preliminary gene expression analysis demonstrated upregulation of genes associated with a Th1 inflammatory response. Based on these initial results we hypothesize that the anti-tumor effect of PSK is mediated through augmentation of an adaptive immune response. The goal of the proposed study is to determine whether PSK induces a Th1 adaptive immune response and, thus, modulates the immunosuppressive tumor microenvironment in the neu-tg mouse. The specific aims of the proposal are to: (1) determine whether PSK enhances activation and maturation of DC1 irt vivo and in vitro, (2) determine whether PSK can stimulate or augment an antigen specific immune response, (3) determine whether PSK can enhance tumor immunogenicity by up regulating immune and co- stimulatory molecules on the tumor as well as decreasing T regulatory cells in the tumor microenvironment. Results from these studies will help us understand the immune modulating effect of PSK and contribute to the clinical translation of this CAM molecule as an anti-cancer agent and/or a vaccine adjuvant.